1. Field of the Invention
The present invention relates to a superconducting quantum interference device (SQUID), more particularly, to a digital SQUID system in which a SQUID therein includes two Josephson junctions, a bias supplied to the SQUID is a pulse current, and a circuit for supplying the bias and a feedback circuit are formed in a single superconducting device chip together with the SQUID.
2. Description of the Related Art
SQUIDs are used for detecting a small current and other signals which can be converted into a current, such as a small magnetic flux, a small voltage or resistance, at a very high sensitivity. For example, SQUID magnetometers are used when measuring a small magnetic flux in a human body, e.g. in a brain or a heart, or a gravity wave, etc.
A SQUID, in general, is classified into two categories: an r.f. SQUID which includes a single Josephson junction, or a d.c. SQUID or two junction SQUID which includes two Josephson junctions. The SQUID system of the present invention pertains to the latter DC SQUID.
In addition, the two junction SQUID can be an analog type SQUID and a digital type SQUID, as described later in detail. The SQUID system of the present invention essentially relates to the digital type SQUID.
Each of the SQUID magnetometers includes a SQUID, a pick-up coil connected to the SQUID and a feedback circuit which supplies a control current to the pick-up coil or to the SQUID for compensating a magnetic flux detected by the pick-up coil.
Harada, et al., in "Experimental Result of QFP (part 2)", Third Symposium of Rikagaku Research Institute, pp. 53-58, Mar. 19, 1986 disclose a quantum flux parametron (QFP) magnetometer which can measure a magnetic flux at a high resolution. The QFP magnetometer, for example, as shown in FIG. 11 of this paper, includes a magnetic comparator consisting of a QFP circuit, having parametron elements which include two Josephson junctions and are connected through a superconducting line and a detector, a pick-up coil connected to the magnetic comparator, and a feedback circuit. The feedback circuit includes an averaging processor connected to the detector, an up/down (U/D) counter connected to the averaging processor, and a digital to analog converter (DAC) receiving a counted value from the U/D counter and supplying an analog converted value to the magnetic comparator. The QFP circuit has a structure shunted by a superconducting loop and does not provide a steady voltage. Accordingly, for example, by using another SQUID, a direction of a current flowing in the loop must be detected, and the circuit constructions of the magnetic comparator and the feedback circuit also differ from those of the present invention. Further, since the feedback circuit is operated at a room temperature, the QFP magnetometer is affected by thermal noise or cross talk and cannot be formed as a single superconductive IC chip.
JPA 62-102176 (Harada, et al., "Magnetometer and Preferred Superconducting Accumulation and Operation Circuit thereof") discloses a QFP magnetometer which is a modification of the QFP magnetometer described in the above paper. Namely, a superconducting feedback circuit for counting transient pulses output from the QFP is disclosed. It is assumed that a time of an output voltage is very short, typically 1 ps to 10 ps, when pulses, each having a long duration, for example, 1 ns are supplied, the QFP magnetometer cannot operate. Further, when pulses, each having a short duraton, are supplied, transient, phenomena must be taken into account, and a stable operation cannot be easily established. The QFP magnetometer, for example, as shown in FIG. 1 of JPA No. 62-102176, includes a pick-up coil 200, an inductor 2, a DC flux parametron (DCFP) circuit 500 having two Josephson junctions 100 and 101, an AC power supply 106 connected to the DCFP circuit 500, an accumulation and operation circuit 600 having two serial-connected SQUIDs 1a and 1b, a U/D counter 300, and DC power supplies 3a, 3b, and 400. The QFP magnetometer requires a relatively large number of power supplies 106, 3a, 3b and 400 and two SQUIDs in addition to the DCFP circuit, and accordingly, suffers from the disadvantage of a complex circuit construction. Further, the above power supplies and the U/D counter are provided under room temperature conditions. As a result, the QFP magnetometer can not be formed as a single superconductive IC chip.
JPA No. 63-149914 (Fujimaki, "Superconducting circuit") discloses a superconducting circuit consisting of a first Josephson junction device J.sub.1, a second Josephson junction device J.sub.2 and a Josephson circuit CCT, as shown in FIG. 1 of JPA No. 63-149914. In the superconducting circuit, by setting different gap voltages, which are voltages within a constant voltage region, of the Josephson devices J.sub.1 and J.sub.2, a rising output pulse current having a high amplitude and a subsequent output pulse current having a lower amplitude than that of the rising output pulse current are supplied to the Josephson circuit CCT to improve the measurement accuracy.
D. Drung, in "Digital feedback loops for DC SQUIDs", Cryogenics 1986 Vol. 26, November, pp. 623-727 discloses a SQUID magnetometer having a digital feedback loop. This paper deals with the feasibility of the integration of a digital feedback loop consisting of a binary U/D counter and a DAC on a chip formed by a Josephson technology, and discloses a technology for obtaining a digital output by supplying an output of the DC SQUID to an ADC integrated on a single chip. However, in the SQUID magnetometer of Drung, a bias which comprises a pulse train having only a positive polarity may be supplied to the SQUID, and thus any fluctuation of the amplitudes of the pulses may cause a fluctuation of an output pulse, producing noise. In other words, a strict stabilization of the amplitude of the bias pulse is required, and thus a complex and high cost bias supplying circuit must be provided.